When a high energy electron or ion beam strikes a sample, photons can be emitted. These emitted photons are also known as cathodoluminescence. The collection and detection of these photons in the wavelength range from UV through visible to IR, can provide a wealth of information about the sample under investigation. However, the signal to be detected can be small and in order to obtain the highest signal to noise ratio, minimize acquisition times and reduce the excitation energy required it is desirable to collect the largest proportion of emitted photons as possible. The most efficient manner of collecting photons emitted from a sample in an electron microscope is by using an optical element which subtends a large solid angle and which detects the photons directly or directs the photons towards a light sensitive element where the photons can be detected and measured; typically, the optical element contains an aperture through which the incident primary electron beam may pass.
Where the optical element used to direct the emitted photons covers such a large solid angle the use of other detectors simultaneously is compromised unless line of sight is provided by sacrificing the solid angle subtended by the optical element. A large solid angle is often required as the photon signal levels is typically many orders of magnitude lower than other signals in the SEM. For many signals in the SEM this is acceptable however, this is not the case for backscattered electrons for several reasons:
1) The emission direction of backscattered electrons and photons is similar and therefore collectors/detectors are required to occupy the same space. Therefore, a large sacrifice in both signals is required so that they may be sensed simultaneously. Backscattered electron detectors are typically mounted to the bottom of the pole piece or inserted immediately below, and the optical element used to direct photons is typically situated between the sample and the backscattered detector, thus obscuring the signal to the back scattered electron detector.
2) Sacrificing part of the optical element's solid angle allows the backscattered detector line of sight to the sample, however the optical element is typically thick (4-20 mm) and the distance between the sample and the backscattered detector large therefore further compromising the solid angle subtended by the backscattered detector in comparison to normal operation.
Detection of photons and backscattered electrons simultaneously is not novel; however, this has only been achieved through large sacrifices in efficiency—typically greater than or equal to 50% reduction in comparison to optimum operation of either detector. This is often unacceptable for many applications. Furthermore, this has only been achieved by the use of two separate instruments.
Publications in the general field include U.S. Pat. Nos. 7,462,839 and 7,707,041 and Japanese Patent Abstract publication no. 11,096,956 by Hiromasa. All references are incorporated herein by reference.